This invention relates generally to railroad-highway grade crossing signaling systems and, more particularly, to such a system employing sychronized sets of flashing incandescent and xenon lamps.
The basic train-activated flashing lights now found at many railroad-highway grade crossings, either alone or in conjunction with automatic gates, have been in use for over 50 years. Although many improvements have occurred in both performance and construction, the aspect presented to the motorist has become well standardized: two incandescent lamps, mounted in reflectors behind red lenses, horizontally aligned at a spacing of 30 inches (0.76 m), against a 20-inch (0.5 m) circular black background, flashed alternately at a rate of 35 to 55 flashes per minute for each lamp. Indeed, the history of this basic pattern can be traced back through the electromechanical wig-wag signal to the motion of a man swinging a red lantern.
Such warnings have generally been found to reduce the occurrence of grade crossing accidents by 60% to 80%. They are now to be found at an estimated 41,600 of the 223,300 public crossings in the United States, accompanied by automatic gates in approximately 9000 cases. Through the years, the railroad supply industry advanced the technology of these lights substantially, particularly in recent times. In addition to improved mountings and reflectors, lenses incorporating more efficient beam patterns and fabricated from nearly-unbreakable polycarbonate materials have been developed. Higher intensity bulbs are now offered, with special reflectorization available, and quartz-halogen lamps are now on the market along with 12-inch (0.3 m) assemblies [compared to the standard 83/8 inch (0.2 m) size].
These changes have generally contributed to greater brightness and improved beam patterns. However, all have occurred within a framework which has severely limited major innovation. Perhaps the most fundamental limitation on conventional warnings is the very low power consumption permitted. It is generally considered necessary (and legally required) that grade crossing protective systems operate from batteries for periods of one to seven days in the event of any failure of commercial power or power system components (such as fuses). This constraint, coupled with the large number of lights commonly used at a crossing (typically four pairs; often three or four times that) led to use of 11-watt bulbs for many years, with 18 watts now standard and 25 watts coming into wide usage. When these figures are compared to the 60 to 150-watt ratings of bulbs used in highway traffic signals, it becomes obvious that adequate intensity can be obtained only through tight focusing of the lights to a narrow beam. This is the course which has been followed, with special roundel design providing a diversion of a limited quantity of light in certain directions to include motorists not located within the main beam. This limitation has been exacerbated by the use of a very deep shade of red, which attenuates light output by approximately 90%. However, the modern use of plastic lenses now permits use of a substantially lighter shade; much tighter manufacturing tolerances are possible than is the case for glass, so roundels can be produced at the lighter limit of existing standards.
Considered in terms of effectiveness as a motorist warning device, serious limitations arise from this technical constraint. The problems are both inherent and practical. In the former category is the challenge of providing an adequately intense light to all positions which a motorist might occupy. Even the use of two or three pairs of lamps, aimed to provide overlapping coverage of the entire approach path, often appears to be marginally adequate. Further, a driver might easily focus his attention upon a pair of lights other than the one appropriate to his position, and be inadequately warned. This difficulty has tended to increase in recent years, as lights have been located further from the road -- both vertically (with cantilevers) and horizontally (beyond highway shoulders). This difficulty is a primary cause of the common (but incorrect) impression that grade crossing lights are inherently less bright than conventional highway traffic signals.
Of comparable importance in practice is the great sensitivity of such a device to misalignment. Whether through misplacement of the bulb, faulty aiming of the assembly, use of an appropriate roundel, or physical movement through accident or malicious vandalism, very little deviation is required to degrade seriously the effectiveness of the warning. The railroad environment is one which makes particularly difficult the attainment and maintenance of optimal conditions. Extremes of weather, continual vibration, sabotage, etc., all make probable that at any given time the lights will deviate somewhat from proper aim.
Thus, one of the fundamental quantitative specifications needed for warning lights is intensity. This is not a simple matter. The brightness required for "adequate" warning depends upon the individual's physical and emotional characteristics, the ambient light level, and the entire visual context. One common criterion for brightness is that the source intensity I.sub.o (candela) appropriate to a viewing distance d (ft), with ambient illuminance L.sub.b (ft-Lamberts), is given by the expression EQU I.sub.o = 6.37 (L.sub.B + 2.92 ) d.sup.2 .times. 10 .sup..sup.-7 (cd)
As an example, "normal daytime conditions" (L.sub.b = 2919 ft-L, or 10,000 cd/m.sup.2) imply I.sub.o = 200 cd to be necessary for a viewing distance of 330 ft. (100 m). Background illuminance can, at times, reach three to four times this value. Further, the intensity required if one seeks to alert as well as inform can increase this value. However, this equation provides a useful starting point, and is readily modified if necessary. (Under night conditions, it is important that intensity not be so great that motorists are bothered or hampered in their actions. Tests in a different but related research activity indicate that a level of 200 to 500 cd is likely to be acceptable for an observer 20 to 50 ft. (6 to 15 m) from the lights.)
A given amount of radiant energy can be utilized as a short, high-intensity flash or a longer pulse at reduced intensity. This suggests the desirability of using very short, very intense pulses in cases for which power efficiency is important. However, the perceived brightness of flashes which are markedly shorter than the response time of the eye (.about.0.1 sec) is basically determined by total flash energy alone, so that no further benefits are obtained for shorter flashes. Numerous studies of this very complex topic confirm that the power efficiency with which a given perceived brightness level can be obtained increases with decreasing duration, down to approximately 0.1 sec; little improvement is found below that interval.
The key then to synthesis of a meaningful advance in railroad crossing signals is the requirement for a short flash duration. Since it is not practical to cycle an incandescent bulb at the pulse durations desired, due to filament heating and cooling times, an alternative is needed. Electromechanical devices, such as rotating beacons, can provide the desired effect. However, considerations of cost, complexity, and maintenance requirements, as well as synchronization and easy adaptation to existing systems, all combine to make this approach impractical.
A more promising approach was disclosed in U.S. Pat. No. 3,390,304 and entails the use of xenon flash-tube (capacitive discharge) lamps. In such lights, the energy stored in a capacitor (1/2CV.sup.2) is released - primarily as visible radiation - by electrical discharge across a xenon-filled gap. The process is readily initiated by an applied "trigger" signal, so that precise timing and synchronization is possible; duration is typically less than 0.001 sec. However, since familiarity is such an important criterion in signaling, the mere substitution of xenon lamp systems for the presently employed incandescent lamp systems would not be prudent. With this factor in mind, xenon lights have been added to conventional flashers in a variety of forms at operating grade crossings. Although generally providing good results, this approach fostered a new concern. The quasi-random pattern produced by asynchronous pairs of xenon and incandescent lamps can be quite attention-getting at some times, but raises serious questions in a motorist warning system wherein uniformity of aspect is a key to rapid identification and understanding on the part of vehicle operations. Such uniformity can be achieved only through assuring that the perceived pattern is the same every time it is encountered.
The object of this invention, therefore, is to establish a more effective signaling system for use at railroad grade crossings.